In recent years, small-sized portable devices have become widespread and, at the same time, a demand for small-sized large-capacity nonvolatile memories has rapidly increased concomitantly with drastic progress of the high-speed information transmission network. Among the nonvolatile memories, a NAND flash memory and small-sized hard disk drive (HDD) have particularly achieved rapid evolution in recording density, and have constituted a large market.
However, it has been pointed out that both of them have reached the limit of recording density. That is, they confront a problem that with the NAND flash memory, an increase in processing cost concomitant with reduction in the minimum line width is remarkable, and with the small-sized HDD, tracking accuracy cannot be sufficiently secured.
Under these circumstances, several ideas for new memories aiming at largely exceeding the limit of recording density are proposed.